Image orthicon with improved beam modulation factor



April 11, 1967 K. H. GEBEL IMAGE ORTHICON WITH IMPROVED BEAM MODULATIONFACTOR Filed July 5, 1962 Few 2 me 7' S M m a a the second of the abovefactors.

United States Patent 3,313,977 IMAGE ORTHICON WITH IMPROVED BEAMMODULATION FACTOR Radams K. H. Gehel, Dayton, Ohio, assignor to theUnited States of America as represented by the Secretary of the AirForce Filed July 3, 1962, Ser. No. 207,441 3 Claims. (Cl. 315-1) Theinvention described herein may be manufactured and used by or for theUnited States Government for governmental purposes without payment to meof any royalty thereon.

The modulation factor in an image orthicon, i.e., the ratio of thesignal to the magnitude of the direct current component, is low, beingof the order of 5 to percent. This greatly impairs the performance ofthe tube at low light levels. The reason for this is that the noiseproduced in the tube is approximately in proportion to the square rootof the total current reaching the electron multiplier stage of the tube.Consequently, a low modulation factor, or, in other words, a relativelylarge direct current component, results in relatively high noisegeneration and a poor signal-to-noise ratio at low signal levels.

It has been experimentally determined that two factors are largelyresponsible for the low modulation factor. One is the wide spread ofelectron velocities in the scanning beam. At low light levels theinformation on the target plate of the tube consists of such a smallcharge that only the very fastest electrons reach the target and areeffective as charge neutralizers. Since only the electrons reaching thetarget produce the beam modulation, the percentage modulation is low. Ameans for remedying this situation by limiting the electron velocitiesin the "scanning beam to a narrow range is described and claimed in myapplication S.N. 5,570, filed Ian. 29, 1960, now Patent No. 3,052,807.The other factor is the secondary emission produced at the edge of thebeam aperture in the electrode that normally performs the dual functionin the image orthicon of final grid in the electron gun and first dynodeof the electron multiplier. These secondary electrons enter the electronmultiplier where they increase the direct current component withoutincreasing the signal, thus lowering the modulation factor.

It is the object of the invention to reduce the effect of Two methodsare dis closed. Considered briefly, one method uses a separate dynodeinsulated from and at slightly higher potential than the 'last electrodeof the gun. The separate dynode has a larger opening than the gunelectrode and the wall of the opening receives the secondary electronsemitted at the edge of gun aperture. The other method provides anadditional electrode surrounding the aperture and held at a slightlyhigher potential than the dynode to attract second electrons and preventtheir entering the electron multiplier.

The invention will be described in more detail with reference to theaccompanying drawings in which:

FIG. 1 illustrates the problem of secondary emission at the edge of thegun aperture in a conventional image orthicon,

FIG. 2 illustrates the separate dynode method of blocking secondaryelectrons from entering the electron multiplier; and,

FIG. 3 illustrates the use of a separate electrode for preventingsecondary electrons from entering the electron multiplier.

Referring to the drawings, the electron gun and electron multiplier of aconventional image orthicon are 3,313,977 Patented Apr. 11, 1967illustrated in FIG. 1. The construction and operation of this type oftelevision pickup tube is well understood in theart and described in theliterature, for example, in an article entitled The Image Orthicon byRose, Weimer and Law appearing in the July 1946, issue of theProceedings of the Institute of Radio Engineers. The electron guncomprises a thermionic cathode 1 having one end coated with a goodemitter of thermal electrons 2 when heated by a heating winding 3 whichreceives current through leads 4. The cathode 1 is surrounded by a beamintensity control electrode 5 having an aperture 6 for the passage ofelectrons emitted by the cathode. An additional electrode 7, sometimesreferred to as the 2nd grid of the electron gun, surrounds the electrode5 and has an aperture 8 in alignment with aperture 6. Electrode 7 ismade positive relative to the cathode so that the electrons are drawnaway from the cathode to form a beam 9 which is controlled by adjustingthe potential of electrode 5 which is normally negative relative to thecathode.

As is well known, the image orthicon has a target electrode (not shown)on which the picture information appears as a variation in positivecharge. The beam 9, which is made up of electrons having just sufiicientvelocity to reach the target, is caused to scan the target byappropriate beam deflecting apparatus (not shown). As the beam scans thetarget it neutralizes the positive charges, with the beam electrons inexcess of those required to neutralize the charge at any point beingreturned toward the electron gun end of the tube as along path 10. Thereturning electron stream, which has been thus modulated in accordancewith the picture information, strikes the face of electrode 7 which actsas the first dynode of an electron multiplier. The resulting secondaryelectrons travel to the second dynode 11, as along path 12, with furtheramplification by electron multiplication taking place in dynodes 13, 14and 15. The final output current is collected by output electrode 16,which has connected to it a load impedance 17 and a coupling capacitor18 for applying the output video signal to an external circuit.

It has been found that considerable secondary emission takes place atthe outer edge of aperture 8 as the rc-v sult of primary electrons fromthe cathode striking electrode 7 at this edge. These secondary electronshave low velocities and are largely in directions transverse to thedirection of beam 9. They therefore behave like the secondary electronsemitted from the face of elec trode 7 when this face is struck byelectrons rejected by the target and, consequently, they travel to thesecond dynode 11 along with the secondary electrons resulting from thetarget return. This increases the total current in the electronmultiplier, and therefore the noise generated, without increasing thesignal and results in a reduoed signal-to-noise ratio.

FIGS. 2 and 3 illustrate two methods, in accordance with the invention,for overcoming the above problem. In FIG. 2, the solution is to use afirst dynode 19 that is separate and electrically insulated from the 2ndgrid 7 of the electron gun. The electrode 19 is maintained slightlypositive relative to electrode 7 and has an aperture 20 large enoughthat primary electrons emerging from aperture 8 cannot strike thedynode. With this arrangement, the secondary electrons emitted at theedge of aperture 8 are attracted to and collected by the more positivewall surface of aperture 20, as shown, and are thus prevented fromentering the electron multiplier.

The arrangement in FIG. 3 is similar in principle to that in FIG. 2except that electrode 7 again performs the dual function of the 2d gridfor the electron gun and first dynode [for the electron multiplier, asin FIG. 1,

with the secondary electron collecting rfun-ction being performed by anadditional small annular electrode 21 surrounding aperture 8. Theelectrode 21 is insulated from and maintained slightly positive relativeto electrode 7. Its inner diameter, as in the case of the diameter ofaperture 20 in FIG. 2, is great enough that the electrode is out of thepath of primary electron-s emerging from the electron gun throughapertrue 8.

I claim:

1. In an image orthicon comprising an elongated electron gun having anaccelerating electrode with an aperture concentric with the longitudinalaxis of said gun for the emission of an axial beam of low velocityelectrons and an electron multiplier having a plurality of cascadedcircular dynodes and the second and succeeding dynodes of which surroundsaid electron gun and are coaxial therewith and in which the firstdynode is in the form of a fiat circular secondary emissive surfacenormal to and concentric with the axis of said gun and surrounding saidaperture, means providing a conductive cylindrical surface of largerdiameter than said aperture but of much smaller diameter than said firstdynode, said cylindrical surface being placed close to said aperture sothat electrons emerging 'from said aperture immediately enter thecylindrical space defined by said surface, the inner diameter of saidcylindrical electrode being large enough that the surface be struck byelectrons originating in said gun and means for maintaining saidconductive cylindrical surface at a positive potential relative to saidaccelerating electrode.

2. In an image orthicon comprising an electron gun having anaccelerating electrode with an aperture for the emission of a lowvelocity electron beam and an electron multiplier having a plurality ofcascaded dynodes the second and succeeding dynodes of which surroundsaid electron gun and are concentric with said aperture, a first dynodefor said electron multiplier providing a secondary emissive surfacelocated near said aperture and surrounding and normal to said beam, saidfirst dynode having a central opening concentric with said aperture andpnoviding a cylindrical conductive wall adjacent to said aperture andconcentric therewith, said opening having a diameter large enough thatelectrons originating in said gun cannot strike said first dynode, andmeans for maintaining said first dynode at a positive potential relativeto said accelerating electrode.

3. In an image orthicon comprising an electron gun having anaccelerating electrode with an aperture for the emission of a lowvelocity electron beam and an electron multiplier having a plurality ofcascaded dynodes the second and succeeding dynodes of which surroundsaid electron gun and are concentric with said aperture and in whichsaid accelerating electrode has a flat circular surface surrounding saidaperture and normal to said beam which serves as the first dynode ofsaid electron multiplier, a cylindrical electrode having an innerdiameter greater than said aperture and an outer diameter much less thanthe diameter of the flat surface of said accelerating electrode, saidcylindrical electrode being concentric with said aperture and adjacentto but I electrically insulated from the flat surface of saidaccelerating electrode, the inner diameter of said cylindrical electrodebeing large enough that electrons originating in said gun cannot strikethe cylindrical electrode and means for maintaining said clyindricalelectrode at a positive potential relative to said acceleratingelectrode.

References Cited by the Examiner UNITED STATES PATENTS 2,747,133 5/1956Weimer 3l365 X 3,197,665 7/1965 Cope 313-293 ELI LIEBERMAN, PrimaryExaminer. HERMAN KARL SAALBACH, Examiner.

S. CHATMON, 111., Assistant Examiner.

1. IN AN IMAGE ORTHICON COMPRISING AN ELONGATED ELECTRON GUN HAVING ANACCELERATING ELECTRODE WITH AN APERTURE CONCENTRIC WITH THE LONGITUDINALAXIS OF SAID GUN FOR THE EMISSION OF AN AXIAL BEAM OF LOW VELOCITYELECTRONS AND AN ELECTRON MULTIPLIER HAVING A PLURALITY OF CASCADEDCIRCULAR DYNODES AND THE SECOND AND SUCCEEDING DYNODES OF WHICH SURROUNDSAID ELECTRON GUN AND ARE COAXIAL THEREWITH AND IN WHICH THE FIRSTDYNODE IS IN THE FORM OF A FLAT CIRCULAR SECONDARY EMISSIVE SURFACENORMAL TO AND CONCENTRIC WITH THE AXIS OF SAID GUN AND SURROUNDING SAIDAPERTURE, MEANS PROVIDING A CONDUCTIVE CYLINDRICAL SURFACE OF LARGERDIAMETER THAN SAID APERTURE BUT OF MUCH SMALLER DIAMETER THAN SAID FIRSTDYNODE, SAID CYLINDRICAL SURFACE BEING PLACED CLOSE TO SAID APERTURE SOTHAT ELECTRONS EMERGING FROM SAID APERTURE IMMEDIATELY ENTER THECYLINDRICAL SPACE DEFINED BY SAID SURFACE, THE INNER DIAMETER OF SAIDCYLINDRICAL ELECTRODE BEING LARGE ENOUGH THAT THE SURFACE BE STRUCK BYELECTRONS ORIGINATING IN SAID GUN AND MEANS FOR MAINTAINING SAIDCONDUCTIVE CYLINDRICAL SURFACE AT A POSITIVE POTENTIAL RELATIVE TO SAIDACCELERATING ELECTRODE.